The present invention relates to characterizing fluid flows, and more particularly to determining directions and velocities within fluid flows, based on light scattered by particles carried by in fluids.
Particle image velocimetry (PIV) is a well-known technique in which laser light sheets and planar imaging are used to measure in-plane velocity components within a plane of a moving fluid. A fluid is seeded with particles, droplets or other discrete light scattering elements. A selected region within the moving fluid is illuminated by carefully timed coherent light pulses and light scattered by the scattering elements is collected to form images. Successive images reveal displacement of elements within the image field, from which flow velocities are determined.
There are several known methods for obtaining information on three dimensions based on PIV imaging. A stereo camera can photograph the planar light sheet to generate three-dimensional vectors on planar domains. Two or more cameras can generate photographic images of the same volume. However, this technique requires matching of images and tracking of individual particles. The resulting three-dimensional particles have an undesirably large depth of focus, i.e. the particle image length in the viewing direction.
Another approach, holographic imaging, involves forming an interference pattern between an object field including light scattered by particles or other scattering elements and a reference field of the coherent light unaffected by the scattering elements. The interfering fields, simultaneously directed onto a holographic film plate or other medium, form an interference pattern having a fringe spacing on the order of micrometers. The interference pattern is photochemically developed into a three-dimensional grating structure. A holographic image representing the light scattering particles is reconstructed by directing a second reference field onto the film plate, to diffract the reference field off the recorded grating structure to produce the holographic image.
In-line holographic techniques, in which the reference beam and scattered energy beam have the same axis, result in a depth of focus too large for satisfactory spatial resolution, particularly in the viewing direction. Large amounts of speckle noise also present a problem. Providing the reference beam at an off-axis angle improves the depth of focus and reduces speckle noise. However, this approach, like in-line holography, requires a relatively low number of relatively large particles. This factor limits the utility of the approach for studying turbulent flows and the spatial resolution is substantially less than that attainable in planar PIV systems.
Measurements in certain types of fluid flows, e.g. turbulent flows, require high accuracy and spatial resolution in three mutually perpendicular directions, and large volumetric domains with displacement directions, amplitudes and velocities expected to vary considerably over the domains. Accordingly, previous holographic systems that require small numbers of relatively large particles, are not well suited for turbulent flow studies.
Therefore, it is an object of the present invention to provide a holographic particle imaging system with enhanced sensitivity for detecting small particles, to facilitate study of fluid flows with higher densities of particles or other light-scattering elements.
Another object of the invention is to provide a system for holographically recording and reconstructing particle images with spatial resolution comparable to resolution in planar photographic PIV systems.
A further object of the invention is to provide an apparatus for recording several particle images on a single holographic plate in a manner that unambiguously distinguishes the images from one another by time and perspective, and an apparatus for reconstructing the images in combinations selected to facilitate derivation of three-dimensional vector data.
Yet another object is to provide a three-dimensional particle imaging system with substantially reduced particle depths of focus in the viewing direction to provide high spatial resolution in three mutually perpendicular directions.